1. Technical Field
This invention relates generally to a method and a slurry for selectively polishing substrates having different materials deposited thereon, and more particularly to such a method and slurry for the chemical mechanical polishing of a substrate having both oxide and nitride films deposited thereon.
2. History of Related Art
Global planarization of topographical features with diverse pattern densities is commonly used in the manufacture of high performance ultra-large scale integration (ULSI) devices. Structures with increasingly smaller device dimensions, high aspect ratio features, denser packaging arrangements, and higher metal-insulator wiring levels demand stringent planarity requirements. Short wave-length photolithography, used to define masks for the subsequent formation of conductive circuit features, imposes additional constraints on vertical topographical variation, requiring that the entire pattern be maintained within the depth of focus of the light beam, normally about .+-.0.3 .mu.m. With increasingly higher numbers of wiring levels, problems with non-planarity become severe and impact the yield and performance of ULSI devices. Shallow trench isolation (STI) in combination with chemical-mechanical polishing (CMP) has been proposed to replace the Local Oxidation of Silicon (LOCOS) technology for &lt;0.5 micron devices for enhanced performance and improved manufacturability. For example, the combination of shallow trench isolation and chemical-mechanical polishing is described in an article titled A VARIABLE-SIZE SHALLOW TRENCH ISOLATION (STI) TECHNOLOGY WITH DEFUSED SIDE WALL DOPING FOR SUBMICRON CMOS, authored by B. Davari et al. in IDEM Technical Digest (1988), pgs. 92-95, and in an article titled SHALLOW TRENCH ISOLATION OF ULTRA-LARGE-SCALE INTEGRATED DEVICES, authored by K. Blumenstock et al and published in Journal of Vacuum Science Technology, January-February 1994, pgs. 54-58.
In the shallow trench isolation process described in the above references, a photoresist is used to define isolation on a silicon nitride-oxide pad. The layers are then dry etched and trenches in a silicon base are defined using a reactive ion etch (RIE). The trenches are filled with low pressure chemical vapor deposition (LPCVD) oxide. A planarization scheme is required to generally remove all oxide from the nitride surface with some nitride remaining on the active areas. In addition, field oxide should be removed above the silicon surface after planarization. This is generally accomplished by RIE etch back, followed by CMP as described by B. Davari in an article titled A NEW PLANARIZATION TECHNIQUE USING A COMBINATION OF RIE AND CHEMICAL MECHANICAL POLISH (CMP), Published on pgs. 61-64 of the 1989 issue of IEDM Technical Digest. The combination of RIE etch back followed by chemical mechanical polishing provides a wider planarization window and also eliminates some remnant non-planarity after RIE. More recently, planarization without the dry etch step, i.e., CMP only, has been proposed by C. Yu, et al. titled Dishing Effects in a Chemical Mechanical Polishing Planarization Process for Advanced Trench Isolation, in the Sep. 14, 1992 issue of Applied Physical Letters, pgs. 1344-1346.
Whichever method is used, the challenge is to achieve global planarization over &gt;200 .mu.m range and over diverse pattern densities without severe dishing as described in the Yu et al. article. In order to achieve global planarization over such a range, a polishing slurry with a high selectivity ratio with respect to oxide and nitride polish rates is required. A slow polish rate for nitride is necessary to ensure that a desired post planarization oxide thickness can be reliably achieved and the silicon subsurface can be protected. Colloidal silica slurries currently in use provide selectivity ratios of 3:1 to 4:1 for oxide to nitride in blanket films. However, this selectivity is reduced on product wafers which have widely varying pattern factors in non-ideal thickness uniformity across the wafer. Frequently, this results in over-polish, under-polish, and non-uniform thickness in the final product. To overcome these problems, slower polishing rates, repeated inspections and several thickness measurements, and stringent process controls are required to assure product quality. All of these measures reduce efficiency, through-put and increase process costs. The slower polish rates and repeated inspection steps also increase the capital investment for polishing and cleaning tools.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a slurry suitable for use in a method by which the oxide to nitride polishing rate selectivity ratio is significantly greater than 4:1. It is also desirable to have such a method in which the initial polishing rate for oxide is relatively high to quickly remove an oxide overlayer, and then the slurry modified to enhance the selectivity ratio between oxide and nitride during final polishing.